Cranes are utilized to lift and transport machinery, products, goods, containers, and other heavy objects within factories and warehouses and to transfer the products to trucks, trains, and/or ships for transportation purposes. A major problem with the movement of products by crane is cable sway that may develop while the product is being transported. When moving a container, for example, to a final destination (position), such as over a tractor-trailer, the container cannot be swinging back and forth. Thus, when the container is placed in location, the cable sway must be essentially eliminated for proper placement of the container. This results in acceleration and deceleration constraints while moving the container to mitigate sway during movement and more importantly at the end location. Additionally, if the swaying motion is not brought under control it may result in a disastrous situation resulting in damage to the container, products, ship, vehicle, crane, dock, or injury to personnel.
Generally, anti-sway control is implemented in a manual or semi-automatic manner by the operator altering the control (e.g. acceleration) of the crane to eliminate sway on lifting and set-down and at the end of travel. Typically, an experienced operator manually controls speed and acceleration to avoid container collisions and provide essentially zero sway upon reaching the container destination, which allows the container to be placed rather quickly. However, in the case of an unskilled operator, the operator must wait for the swaying to stop before lowering the container into the designated location. In the case of an inexperienced operator, the waiting period can add as much as thirty percent to the cycle time to move the goods, resulting in lost productivity and increased costs. This time becomes especially costly in sea transport because when a ship is at port longer than necessary due to the increase in movement time it results in additional demurrage charges. Furthermore, delays during loading and unloading operations can tie up limited docking space and prevent other ships from docking and carrying out loading and/or unloading activities. Prolonged sway also creates a greater hazard of the container and spreader striking objects and/or personnel, resulting in a more dangerous condition than if the container sway is controlled.
Container cranes utilized to load and unload containers from ships typically have a horizontal beam that supports a movable trolley. Attached to the trolley and suspended by cables is a hook or spreader that attaches to a container. The container may be empty or it may be heavy with shipped goods. The spreader is lowered over the container and the spreader locks grab onto the four corners of the container. The spreader is now attached to the container and raises the container allowing for clearance of other containers, ship structural components, personnel, etc. The container is brought shore side, for example and placed on the ground or on a truck. The operator has full control of the container in the two dimensional plane of movement. When riding in the cab attached to the trolley, the operator looks down though a window in the floor of the cab and positions the container/spreader. The operator controls raising, lowering, and horizontal travel and guides the container to/from the ship. The constraints here include motor power and braking power as set by drive limits, mechanical and structural load limits, and acceleration/deceleration limits such that the operator must survive transit and not become injured or excessively fatigued.
Underway replenishment, or ship-to-ship transfer on the open sea, requires sufficient ship spacing, slow container transit speed, and sufficiently calm sea states along with expert operator control. A cable is attached to each ship and a cargo container or a net containing cargo is attached to the cable. The motorized movement of the cable transfers the cargo from one ship to the other. A controller is also employed to regulate tension on the cable to accommodate ship movement due to propulsion, wind, current, waves, and other disturbances. The controller functions to insure that cable tension does not get too tight to snap the cable or damage the ship and to insure that the cable does not become too loose causing the load to sag and/or become lost or damaged from water or wave action.
An expert crane operator with years of experience is able to effectively control sway by manually compensating for sway while moving the container or empty spreader quickly in a planned direction and speed. Expert crane operators are limited in number and often represent a scarce and costly resource, thus, it is desirable to have less skilled operators perform with the same proficiency as skilled crane operators. This can be accomplished by improving the crane control system capabilities through the addition of anti-sway control that enhances the operator's control. The anti-sway control can augment the manual control of the operator and provide cues to the operator and/or modify the operator's commands of speed, acceleration, and deceleration to reduce or eliminate sway. Conventional anti-sway control techniques operate by using a reverse deceleration scheme. For example, the operator accelerates the crane to move the container away from the ship. The anti-sway control measures this acceleration and provides the same amount of acceleration but in a reverse direction to decelerate the swing. Some controls use proportional integral control while others use speed or position, etc. to facilitate this control. The two-dimensional position can be controlled. However, conventional systems are not able to control a force coming from a third direction, such as wind coming from another angle and thus cannot control the movement that is out of plane. Therefore, there is a need to provide high speed, optimum time, and safe transit of cargo from ship to/from shore for container cranes and for cargo transfer in underway replenishment while compensating for out of plane forces acting on the crane and with non-expert operators.